: Recent evidence indicates that estrogen treatment in ovariectomized rats enhances performance on memory tasks. Similarly, human observations suggest a beneficial effect of estrogen treatment on cognitive function in Alzheimer's disease. However, those cellular targets of gonadal hormones that are directly involved in memory processes are ill defined. The only solid observations are that systemic hormonal manipulations result in changes in the density of spines and alterations in the intensity of immunostaining for NMDA receptor of hippocampal CAl pyramidal cells. However, hippocampal principal neurons themselves do not, only a small population of interneurons contains nuclear estrogen receptor. On the other hand, neurons in subcortical areas, including the medial septum diagonal band of Broca (MSDB), supramammillary area (SUM), and median raphe (MR) contain nuclear estrogen receptors, and these structures are associated with the generation/regulation of hippocampal theta activity and long term potentiation. Furthermore, hippocampal theta activity, which is greatly influenced by the changing levels of circulating estrogen, in conjunction with long-term potentiation is believed to be involved in memory processes. Therefore, the hypothesis that estrogen, in addition to influencing the hippocampus directly, regulates mnemonic processes by affecting these subcortical areas will be tested by experiments designed to examine the effects of intra-MR, -MSDB, and -SUM administration of estrogen in ovariectomized rats on: 1) changes in dendritic spine density of hippocampal CAl pyramidal cells; 2) mRNA and peptide levels of ionotropic glutamate receptors in the hippocampus; 3) hippocampal theta activity.